These 0db power supplies have no fans to cool them so they must all find a way to combat the heat build up, especially under a load. A standard PSU generally has an intake fan and an exhaust fan, the combination of which create an air flow which pulls air from the case, cools the power supply's heatsinks and then leaves the PSU enclosure. This process not only keeps the PSU cool but is an effective way to remove hot air from the case's main compartment.
[image courtesy of PC Power and Cooling]
With no fans these power supplies still need a way to disperse heat, otherwise they would not be able to efficiently lose power. A PSU rated at 300w, generally speaking, produces that amount at room temperature, but once things heat up the amount of power available drops considerably [see chart]. This is going to be a real problem for any power supply that cannot cool itself properly.
Due to the lack of fans, these power supplies have to be a little more creative in that ways in which they remove heat. The most noticeble of these is the use of an aluminum case. Instead of being made of a thin steel enclosure each of the power supply in this roundup is encased in relatively thick aluminum. This aluminum housing is usually finned or in some way vented to allow heated air to get away from the inside of the PSU, which would be negatively affected by the hot conditions.
Some fanless PSUs, specifically the Thermaltake, take advantage of heatpipes and exterior copper fins to transfer the heat from the interior aluminum heat sinks to the outside. Heatpipes are an very effective way to transfer heat but without these fins they are good deal less effective. The issue becomes where to place the fins and the only answer is outside the power supply. This can be an issue depending on your case, its interior, and where you put your system, but we'll look into this more later.
As with any other power supply a fanless one must be able to furnish your computer with stable power. Computers have stringent voltage requirements that the PSU must be able to meet. A +12V rail must operate within 5% of 12V, so 11.40V to 12.60V. A +5V rail must be kept between 4.75V and 5.25V (+/-5%), and your +3.3V has to be between 3.14V and 3.47V. These limits are pretty important and any power supply that can't provide power within their bounds is useless, no matter how quiet it is.
The last issue is going to be how much power your computer requires. Certain P4 processor can require up to 120W when at 100% capacity. This generally does not happen for more than a few seconds at a time but your PSU has to be able to handle the load of a full computer. This includes your processor, drives, video card, cooling, those lights you installed, etc. Considering the draw of certain video cards you have to be careful what power supply you use. For example nVidia originally recommended you use a 480+ watt PSU with its GeForce 6800 Ultra. They were probably playing it safe but its better to be on the safe side. Fanless PSUs are not for everyone so do the math before you pick up one and you won't throw away a lot of money on an ATX12V Ver. 2.01 compliant paper weight.